On this day January 7, 1610

Italian astronomer Galileo Galilei first observed three of Jupiter’s moons through his telescope: Io, Europa, and Callisto.

Io moon of Jupiter

On 7 January 1610 Galileo observed with his telescope what he described at the time as “three fixed stars, totally invisible by their smallness”, all within a short distance of Jupiter, and lying on a straight line through it.

Observations on subsequent nights showed that the positions of these “stars” relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars.

On 10 January Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days he concluded that they were orbiting Jupiter.

He had discovered three of Jupiter’s four largest satellites (moons): Io, Europa, and Callisto.

He discovered the fourth, Ganymede, on 13 January. Galileo named the four satellites he had discovered Medicean stars, in honour of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and Cosimo’s three brothers. Later astronomers, however, renamed them Galilean satellites in honour of Galileo himself.

The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jupiter system and Io on February 28, 2007.

During the encounter, numerous distant observations of Io were obtained. These included images of a large plume at Tvashtar, providing the first detailed observations of the largest class of Ionian volcanic plume since observations of Pele’s plume in 1979.

New Horizons also captured images of a volcano near Girru Patera in the early stages of an eruption, and several volcanic eruptions that have occurred since Galileo mission.